Optical systems for laser arrays

ABSTRACT

Surface emitting laser arrays with intra-cavity harmonic generation are coupled to an optical system that extracts harmonic light in both directions from an intra-cavity nonlinear optical material in such a way that the focusing properties of the light beams are matched.

TECHNICAL FIELD

The disclosure is generally related to laser arrays. In particular it isrelated to optical systems for efficiently focusing harmonic lightemitted from a laser array.

BACKGROUND

Laser arrays are promising light sources for a number of applicationssuch as projection displays and specialty lighting. Compared to lamps orlight emitting diodes, for example, diode laser arrays offertremendously increased brightness with long life and high reliability.Light generated by laser arrays can have high enough brightness to takeadvantage of nonlinear optical processes that convert light from onewavelength to another. For example, second harmonic generation (alsocalled “frequency doubling”) is a nonlinear optical process by whichlight at infrared wavelengths can be converted to visible wavelengths.

One way to make visible light is to focus a high intensity infraredlaser beam onto a suitable nonlinear optical material as firstdemonstrated in 1961 by Franken. More recently scientists have shownthat second harmonic light may be generated by placing a nonlinearmaterial inside a laser cavity; i.e. between two mirrors that form theends of a laser. Second harmonic generation inside a laser offers higherconversion efficiency than can be achieved externally. An example of adevice using this approach is the Novalux, Inc. (Sunnyvale, Calif.)Novalux Extended Cavity Surface Emitting Laser (NECSEL) array thatproduces light at red, green and blue wavelengths via intra-cavitysecond harmonic generation.

Surface emitting diode lasers emit light perpendicular to the surface ofthe semiconductor substrate on which they are fabricated. The laserstructure includes a high reflector built on the substrate. An opticalgain section is built between the high reflector and an output coupler.When the laser is designed for intra-cavity harmonic generation theoutput coupler is made to be a high reflector at the fundamentalfrequency and transparent at the harmonic frequency.

Consider the situation in which second harmonic light is generatedinside a diode laser cavity in a nonlinear optical material. The highreflector on the substrate is likely to be transparent at the secondharmonic wavelength. Second harmonic light passing through the outputcoupler can be easily directed to the application at hand. Secondharmonic light passing through the substrate high reflector, however, islost to absorption in the semiconductor substrate.

Second harmonic light traveling toward the substrate high reflector canbe saved by inserting a dichroic beam splitter in the laser cavity. Adichroic beam splitter reflects light at the second harmonic wavelengthwhile transmitting light at the fundamental laser wavelength. Mirrorsexternal to the laser cavity may be used to direct the second harmoniclight in any direction, including quite usefully, parallel to the secondharmonic light that passed through the laser output coupler. In this wayas much visible light as possible may be extracted from an infrareddiode laser array featuring intra-cavity second harmonic generation.

Although the method of extracting second harmonic light from a diodelaser cavity through an output coupler in one direction and a dichroicbeam splitter in the opposite direction is effective in terms of totallight power recovered, it may not be optimum when beam focusing isconsidered. Light from the two sources is focused at different planeswhen both are focused by a single lens. The difficulty comes from thetwo different path lengths traveled by second harmonic light from itswaist inside the laser cavity. Depending upon how external mirrors arearranged, the path via the output coupler to a focusing lens may beshorter than the path via the dichroic beam splitter.

Surface emitting diode laser arrays with intra-cavity second harmonicgeneration would benefit from an optical system that not only extractedsecond harmonic light in both directions from the intra-cavity nonlinearoptical material, but did so in such a way that the focusing propertiesof both light beams were matched.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are heuristic for clarity.

FIG. 1 shows a diode laser array with light beams propagating away fromit in two perpendicular directions.

FIG. 2 shows a diode laser array with light beams propagating away fromit in one direction.

FIG. 3 shows the structure of an extended vertical cavity surfaceemitting diode laser with an intra-cavity second harmonic generationsection.

FIGS. 4A and B show an extended vertical cavity surface emitting diodelaser with a dichroic beam splitter to extract second harmonic light.

FIG. 5 shows a light emitter array and a retro-reflector.

FIG. 6 shows a two different ways to realize a retro-reflecting opticalelement.

FIG. 7 shows an extended vertical cavity surface emitting diode laserwith a dichroic beam splitter and an external lens and mirror.

FIG. 8 shows a lens and mirror with a light emitter array.

FIG. 9 shows an array of lenses and a mirror with a light emitter array.

DETAILED DESCRIPTION

Arrays of light emitters, such as diode laser arrays, are useful aslight sources for projection displays. For example, Novalux, Inc.(Sunnyvale, Calif.) NECSEL arrays provide high brightness at red, greenand blue wavelengths.

FIG. 1 shows a diode laser array with light beams propagating away fromit in two perpendicular directions. In the figure, array 105 containsseveral diode lasers that radiate light beams such as light beams 110,111, 112, etc, and 120, 121, 122, etc. Each pair of light beams, e.g.110 and 120, is generated by a single external cavity diode laser. Beams110, 111, 112, etc are emitted through their respective diode laser'soutput couplers. Beams 120, 121, 122, etc, on the other hand, arecoupled out of their respective diode laser's cavities by a dichroicbeam splitter which is not shown in the figure. Beams 120, 121, 122,etc, propagate out of the page, toward the reader.

The diode lasers are spaced apart by distance, d, and the 1/e² diameterof the waist of each light beam is equal to 2w₁. In a typical array 24NECSELs are spaced apart by d˜350 microns and each laser emits aGaussian beam with a waist diameter of approximately 70 microns. Neitherthe number of emitters nor the spacing nor the beam diameter is ofparticular importance here. The light beams diverge as they propagate.The divergence angle is a function of the beam waist and the wavelengthof the light.

A diode laser array such as the one shown schematically in FIG. 1 may beused as a light source for a projection display. In a display system,light from a light source is formed into an image by a light modulator.The brightness of the displayed image depends in part on how efficientlylight from the light source illuminates the modulator. For example, iflight from the light source is improperly focused on the modulator, ornever reaches the modulator, then the displayed image will not be asbright as it otherwise might be. The diode laser array of FIG. 1 couldbe improved for display applications if light beams 120, 121, 122, etc,were redirected so that they propagated between, parallel to, in thesame plane, and with the same divergence as, light beams 110, 111, 112,etc. This scenario is illustrated in FIG. 2.

FIG. 2 shows a diode laser array with light beams propagating away fromit in one direction. In the figure, array 205 contains several diodelasers that radiate light beams such as light beams 210, 211, 212, etc,and 220, 221, 222, etc. Beams 210, 211, 212, etc are emitted throughtheir respective diode laser's output couplers. Beams 220, 221, 222,etc, on the other hand, are coupled out of their respective diodelaser's cavities by a dichroic beam splitter which is not shown in thefigure. Beams 220, 221, 222, etc, are directed into their respectivepositions between, parallel to, and in the same plane as light beams110, 111, 112, etc, by optical components that are not shown in FIG. 2,but are discussed below. Furthermore light beams 220, 221, 222, etc,have been manipulated so that their divergence matches that of lightbeams 210, 211, 212, etc. That way all of the light beams from the diodelaser array can be focused by a lens to a waist lying in one plane.

FIG. 3 shows the structure of an extended vertical cavity, surfaceemitting diode laser with an intra-cavity second harmonic generationsection. In the figure, a gain section and high reflecting mirror arefabricated in semiconductor process layers 310 on a substrate 305.Nonlinear optical structure 315 and output coupler 320 are placed sothat a laser cavity is formed with the nonlinear optical structure 315lying within the cavity. Nonlinear optical structure 315 may be madefrom, for example, a nonlinear optical crystal or an engineerednonlinear optical material such as periodically poled lithium niobate.Dichroic beam splitter 330 reflects second harmonic light (e.g. visiblewavelengths) generated in nonlinear optical structure 315 forming lightbeam 355. Dichroic beam splitter 330 transmits light at the fundamentallaser wavelength (e.g. infrared). Light beam 350 represents secondharmonic light that passes through output coupler 320.

Dotted lines 335, 340 and 345 represent reference planes used indiscussion herein. In particular reference plane 340 marks the locationof the waist of light beams 350 and 355. In practice the waist may notbe located exactly as drawn in the figure; however, the waist liesinside the laser cavity and its location can be measured or calculatedfrom laser cavity parameters. Reference plane 335 represents an outputface of the diode laser in the direction defined by light beam 350 whilereference plane 345 represents an output face of the diode laser in thedirection defined by light beam 355.

FIGS. 4A and B show an extended vertical cavity surface emitting diodelaser with a dichroic beam splitter to extract second harmonic light. InFIG. 4A diode laser 410 is the same as the diode laser of FIG. 3 as seenfrom a vantage point in the plane of FIG. 3 and looking perpendicular tothe direction of propagation of laser beam 350. In FIG. 4A, dichroicbeam splitter 430 (corresponding to dichroic beam splitter 330 in FIG.3) is shown extended beyond diode laser 410. The function of beamsplitter 430 could be achieved by two smaller beam splitters as its beamsplitting properties are only needed in places where light beams areincident upon it. Arrow, dot and cross 455, 460 and 465 respectively allrefer to a beam of second harmonic light extracted from the cavity ofdiode laser 410. This beam corresponds to light beam 355 in FIG. 3 as itis reflected in various directions. In particular FIG. 4B showsschematically how light beam 460 is reflected in direction 465 by retroreflector 470. Dotted line 435 represents a reference plane used indiscussion herein.

In FIG. 4B retro reflector 470 directs light beam 460 back toward beamsplitter 430 which in turn directs the beam in direction 455. The viewof FIG. 4B is one in the plane of FIG. 4A and looking parallel to beams450 and 455. It can be seen by comparison to FIGS. 1 and 2 thatretro-reflector 470, in combination with beam splitter 430, partiallyachieves the result shown in FIG. 2: two second harmonic light beamsparallel to one another and in the same plane with beams from otherdiode lasers in an array.

The result is only partially achieved because light beams 450 and 455are not the same size at plane 435. Both beams diverge from a waistinside diode laser 410, but beam 455 has traveled farther by the time itreaches plane 435. These two beams cannot conveniently be focused to acommon waist by a single lens. In some cases, for example if theRayleigh range is long compared to the extra distance traveled by beam455, the system shown in FIGS. 4 and 5 may be sufficient.

FIG. 5 shows a light emitter array and a retro-reflector. In the figurelight beams 507, 508, 509, etc, drawn as solid lines radiate from array505. Retro reflector 520 redirects the beams back toward array 505 asbeams 510, 511, 512, etc. These beams, drawn as dotted lines, areinterleaved approximately half way between the beams radiating away fromthe array. FIG. 5 represents a generalization of the arrangement shownin FIG. 4B from two beams to several beams. As in FIG. 4 the retroreflected beams diverge over the distance they travel to and from theretro reflector. In some cases, this may represent an adequate solutionwhile in others additional optics to control beam divergence may berequired.

Conceptually, what is needed is a way to place a retro reflector at thewaist of the second harmonic light beam in an external cavity diodelaser. In practice, this may be accomplished by using a lens and amirror to form a virtual retro reflector. FIG. 6 shows a two differentways to realize a retro-reflecting optical element.

FIG. 6A shows retro reflector 610 with light rays 615 and 620 incidentupon it. Retro reflector 610 may be realized as a cube corner reflector,roof prism or similar element with plane reflecting surfaces. Looking ata retro reflector one sees not one's mirror image, but rather one'simage as seen in a photograph. Put another way light rays incident uponretro reflector 610 are returned as if folded back at reference plane670. If a Gaussian beam is focused on retro reflector 670 so that thebeam waist lies in plane 670, the return beam will diverge such that itoverlaps the incoming beam exactly for all distances away from thereference plane. The retro reflecting properties of 610 exist for lightbeams arriving at a range of angles away from (and including) the normalto plane 670.

FIG. 6B shows an alternative to retro reflector 610. In FIG. 6B lightrays 660 and 665 are incident upon lens 650. Lens 650 is placed onefocal length way from mirror 655. Dotted line 680 lies on the symmetryaxis of lens 650. Dotted line 675 is a reference plane perpendicular toaxis 680 and located one focal length away from lens 650 on the oppositeside of the lens from mirror 655. Light rays 660 and 665 are retroreflected at plane 675 by the combination of lens 650 and mirror 655. Ifa Gaussian beam is focused to a waist at plane 675 the return beam willdiverge such that it overlaps the incoming beam exactly for alldistances away from the reference plane. It can be seen that the lensand mirror system provides the same retro reflecting characteristic asthe cube corner reflector with one important difference: there is nophysical structure at reference plane 675 of the lens and mirror system.Therefore the lens and mirror system of FIG. 6B can be used to, ineffect, place a retro reflector at the waist of the second harmoniclight beam in an external cavity diode laser.

FIG. 7 shows an extended vertical cavity surface emitting diode laserwith a dichroic beam splitter and an external lens and mirror. Thesystem illustrated in FIG. 7 is similar to that shown in FIG. 3 with theaddition of an external lens and mirror. The system shown in FIG. 7effectively retro reflects a second harmonic light beam that has beencoupled out of a diode laser cavity by a beam splitter back through thecavity. The waist of the retro reflected beam coincides with the waistof a second harmonic light beam that propagates from a nonlinear opticalsection toward an output coupler. The divergences of the two secondharmonic light beams are therefore matched.

In the figure, a gain section and high reflecting mirror are fabricatedin semiconductor process layers 710 on a substrate 705. Nonlinearoptical structure 715 and output coupler 720 are placed so that a lasercavity is formed with the nonlinear optical structure 715 lying withinthe cavity. Nonlinear optical structure 715 may be, for example, aperiodically poled lithium niobate structure, a nonlinear opticalcrystal or an engineered nonlinear optical material. Dichroic beamsplitter 730 reflects second harmonic light (e.g. visible wavelengths)generated in nonlinear optical structure 715 toward lens 760. Dichroicbeam splitter 730 transmits light at the fundamental laser wavelength(e.g. infrared). Light beam 750 represents second harmonic light thatpasses through output coupler 720.

Dotted lines 735, 740 and 745 represent reference planes used indiscussion herein. In particular reference plane 740 marks the locationof the waist of light beam 750. Reference plane 735 represents theoutput surface of a diode laser array while reference plane 745represents a side surface of the array. It is difficult or impossible toplace a bulk optical component closer to the diode laser than referenceplanes 735 or 745.

Lens 760 is placed near the diode laser such that the optical pathdistance from the lens to waist reference plane 740 is one focal length.This distance is the sum of the distance from the lens to beam splitter730 and the distance from the beam splitter to waist reference plane740. Mirror 765 is placed one focal length away from lens 760 on theopposite side of the lens from the diode laser. The distance from thelens to reference plane 745 is denoted “f-x” in the figure to show thatit is less than one focal length.

It can be seen by comparison of FIGS. 7 and 6B that lens 760 and mirror765 are placed in the retro reflective arrangement of FIG. 6B. Referenceplane 675 in FIG. 6 corresponds to waist reference plane 740 in FIG. 7.In FIG. 7, lens 760 is illustrated as having its axis coincident withthe axis of a light beam coupled out of the laser cavity by a dichroicbeam splitter. However, lens 760 may also be displaced perpendicular tothe light beam, for example in a direction perpendicular to the plane ofthe figure. In this way the return light beam can be made parallel tolight beam 750 rather than coincident with it.

The virtual retro reflector of FIG. 6B may be applied to an array ofdiode lasers of the type illustrated in FIGS. 3 and 7. FIG. 8 shows alens and mirror with a light emitter array. The system illustrated inFIG. 8 combines the virtual retro reflector properties illustrated inFIGS. 6B and 7 with the interleaving of light beams illustrated in FIG.5.

FIG. 8 shows a light emitter array and a lens and mirror combinationacting as a virtual retro-reflector. In the figure light beams 807, 808,809, etc, drawn as solid lines radiating from array 805. Lens 820 andmirror 830 redirect the beams back toward array 805 as beams 810, 811,812, etc. These beams, drawn as dotted lines, are interleavedapproximately half way between the beams radiating away from the array.FIG. 8 represents a generalization of the arrangement shown in FIG. 7from one diode laser to an array of many such lasers. Mirror 830 isplaced one focal length away from lens 820 on the opposite side of thelens from array 805. Lens 820 is placed a distance f-x away from theside surface of array 805. (The side surface of array 805 is analogousto reference plane 745 in FIG. 7 which represents the side surface of adiode laser.) The distance from lens 820 to the plane containing thedesired location of retro reflected beam waists (analogous to referenceplane 740 in FIG. 7) is the focal length of the lens. Matched secondharmonic beams in the system of FIG. 8 are radiated in a directionperpendicular to the plane of the page.

The system of FIGS. 7 and 8 extracts second harmonic light radiated inboth directions from the intra-cavity nonlinear optical material in aNECSEL or similar structure and it does so in such a way that thefocusing properties of both light beams are matched. Furthermore, thedifference in path length traveled between light that goes directly toan output coupler and light that traverses the lens—mirror virtual retroreflector is helpful for speckle reduction in display applications.

FIG. 9 shows an array of lenses and a mirror with a light emitter array.This system represents an alternative embodiment of the same principlesapplied in FIG. 8. In FIG. 9 light beams 907, 908, 909, etc, drawn assolid lines radiating from array 905. Lenses 920, 921, 922, etc, andmirror 930 redirect the beams back toward array 905 as beams 910, 911,912, etc. These beams, drawn as dotted lines, are interleavedapproximately half way between the beams radiating away from the array.FIG. 9 represents a generalization of the arrangement shown in FIG. 7from one diode laser to an array of many such lasers. Mirror 930 isplaced one focal length away from lenses 920, 921, 922, etc, on theopposite side of the lenses from array 905. Lenses 920, 921, 922, etc,are placed a distance f-x away from the side surface of array 905. (Theside surface of array 905 is analogous to reference plane 745 in FIG. 7which represents the side surface of a diode laser.) The distance fromlenses 920, 921, 922, etc, to the plane containing the desired locationof retro reflected beam waists (analogous to reference plane 740 in FIG.7) is the focal length of the lenses. Matched second harmonic beams inthe system of FIG. 9 are radiated in a direction perpendicular to theplane of the page.

The optical systems disclosed herein offer efficient light extractionfrom diode laser arrays for display applications. Second harmonic lightgenerated in both directions from an intra-cavity nonlinear opticalmaterial is recovered by a virtual retro reflector so that the focusingproperties of the light are optimized. Further, speckle is reduced byretro reflected path lengths that may be longer than the coherencelength of diode lasers in an array.

All of the optical systems disclosed herein also apply to arrays oflasers that are not diode lasers. For example, an array of opticallypumped lasers would benefit from the optical systems described herein inthe same way that diode lasers do. Furthermore, it is unnecessary forthe harmonic light generated in the laser cavities to be second harmoniclight. All of the optical systems described herein apply equally toother harmonics or light produced by sum frequency generation,difference frequency generation, optical parametric oscillators,four-wave mixing, and/or other nonlinear optical processes. The methodof producing gain and the frequency of the light created in a nonlinearprocess may be selected to suit a particular application.

As one skilled in the art will readily appreciate from the disclosure ofthe embodiments herein, processes, machines, manufacture, means,methods, or steps, presently existing or later to be developed thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, means, methods, or steps.

The above description of illustrated embodiments of the systems andmethods is not intended to be exhaustive or to limit the systems andmethods to the precise form disclosed. While specific embodiments of,and examples for, the systems and methods are described herein forillustrative purposes, various equivalent modifications are possiblewithin the scope of the systems and methods, as those skilled in therelevant art will recognize. The teachings of the systems and methodsprovided herein can be applied to other systems and methods, not onlyfor the systems and methods described above.

In general, in the following claims, the terms used should not beconstrued to limit the systems and methods to the specific embodimentsdisclosed in the specification and the claims, but should be construedto include all systems that operate under the claims. Accordingly, thesystems and methods are not limited by the disclosure, but instead thescope of the systems and methods are to be determined entirely by theclaims.

1. An optical system comprising: a laser from which a harmonic lightbeam is coupled by an intra-cavity dichroic beam splitter; a lens placedone focal length away from the waist of the harmonic light beam; and, amirror placed one focal length away from the lens on the opposite sideof the lens from the waist.
 2. The system of claim 1 wherein the lensand the mirror redirect the harmonic light beam coupled by theintra-cavity dichroic beam splitter so that the light beam propagatesparallel to, in the same plane as, and with the same divergence as, aharmonic light beam coupled from the laser by the laser's outputcoupler.
 3. The system of claim 1 wherein the lens and the mirror form avirtual retro reflector at the waist of the harmonic light beam.
 4. Thesystem of claim 1 wherein the laser is a diode laser.
 5. The system ofclaim 4 wherein the diode laser is an extended vertical cavity surfaceemitting diode laser.
 6. The system of claim 1 wherein the harmoniclight beam is a second harmonic light beam.
 7. The system of claim 1wherein the harmonic light beam is generated within an engineerednonlinear optical material within the cavity of the laser.
 8. The systemof claim 7 wherein the engineered nonlinear optical material isperiodically poled lithium niobate.